Question: which railway electrification system has the highest frequency?
Paraphrase: Of all railway electriciation systems, which uses the highest frequency?
SPARQL (Wikidata): select ?ent where { ?ent wdt:P31 wd:Q388201 . ?ent wdt:P2144 ?obj } ORDER BY DESC(?obj)LIMIT 5 
Answer: 25 kV AC railway electrification, 20 kV AC railway electrification, 12.5 kV, 60 Hz AC railway electrification, 25 kV, 60 Hz AC railway electrification, 20 kV, 60 Hz AC railway electrification